The proposed research is concerned with the detailed characterization of the ultrastructure of the early carious lesion, with particular emphasis on the outer layers of enamel. The major goals involve the three dimensional characterization of both sound and carious enamel, the identification of paths of transport through the enamel, the determination of sites for the initiation of the carious attack, the chemical and structural identification of crystallites both amenable and resistant to carious attack, and the determination of the influence of crystallite size, packing and orientation on the dissolution process. Both "in vitro" and "in vivo" lesions will be studied using the techniques of stereo high voltage electron microscopy, scanning electron microscopy, and special techniques of micro spot selected area diffraction and micro spot energy dispersive analysis.